1. Field of the Invention (Technical Field)
The present invention relates to a procedurally simple, low cost, potentially continuous process for production of boron nitride powders exhibiting smooth spherical morphology, spheroidal particles with “bladed” surface morphology, spheroidal particles with protruding “whiskers,” and fully “bladed” particles with platelet morphology, and particles having turbostratic or hexagonal crystal structure. The process utilizes an aerosol assisted vapor phase synthesis (AAVS) wherein non-aqueous based or partially aquated aerosol droplets containing boron precursors are nitrided through a boron oxide nitride carbide intermediary composition to form spherical and modified spherical boron nitride powders. The process can be achieved through use of an aerosol assisted vapor phase reactor system (AAVRS) or related spray/mist pyrolysis reactor system. It has significant use in preparation of the preferred spheroidal boron nitride powders for use in the microelectronic, polymer, and cosmetic industries as well as in traditional ceramic markets (e.g., aerospace and automotive products).
2. Background Art
Boron nitride (BN) is a commercially produced refractory non-oxide ceramic material whose properties are highly dependent on its crystalline structure The most common structure for BN is a hexagonal crystal structure (h-BN). This structure is similar to the graphitic structure of carbon, and consists of extended two-dimensional layers of edge-fused six-membered (BN)3 rings The layers arrange so that B atoms in the rings in one layer are above and below N atoms in neighboring layers and vice versa (i.e., the rings are shifted positionally with respect to layers). The intraplanar B—N bonding within layers in the fused six-membered rings is strongly covalent while the interplanar B—N bonding is weak, similar to graphite. The layered, hexagonal crystal structure results in anisotropic physical properties that make this material unique in the overall collection of non-oxide ceramics.
From the commercial standpoint, h-BN is typically obtained as a powder, most often from multi-step processes employing boric oxide, sodium borate, or boric acid (as the boron raw material) and urea, melamine, and/or ammonia (as the nitriding source). In its powder form, BN can be processed by classical powder-forming methods into simple and complex shapes. Such powders are often hot-pressed in the fabrication of finished articles. Since it is soft, the hot pressed, processed bodies can be easily machined. BN is also obtained by chemical vapor deposition (CVD) growth, referred to as “pyrolytic” BN.
Pyrolytic BN is considered the most typical form of BN in the industry, given the absence of binders and improved crystallinity and grain features. As a result, unless otherwise indicated, properties of BN described in these background materials are representative of pyrolytic BN. Under standard solid state synthesis conditions, BN is typically obtained as a mixture of meso-graphitic and turbostratic modifications that contain varying degrees of disorder of the ideal hexagonal BN structure (h-BN). Fully ordered h-BN is only obtained with careful attention to synthetic detail. (Paine, RT, Narula, CK. Synthetic Routes to Boron Nitride. Chem. Rev. 90 73–91, 1990.)
All of the syntheses are driven by the thermodynamic stability of BN (in the absence of oxygen and moisture, BN is stable above 2000° C. in N2) and the reducing nitridation conditions that remove impurities. (Paine RT, Narula, CK. Synthetic Routes to Boron Nidride. Chem. Rev. 90 73–91 1990.) Carbothermal reduction conditions can also be employed to remove impurity oxygen. Commercial powder producers manipulate reaction conditions in order to achieve target powder purity, grain size, sinterability, and crystallinity. These features, in turn, influence powder processibility and finished product performance. It is important to note that commercial powders are usually obtained with primary particles having a platelet morphology, a macroscopic manifestation of the inherent crystal structure of h-BN, or as primary particle agglomerates having irregular morphology.
Commercial applications for h-BN are well established in several traditional ceramic markets. In particular, the high temperature stability, chemical inertness, lubricity, electrical resistivity and thermal conductivity make BN powders ideal for fabrication of products used in aerospace, automotive and microelectronic products, including large crucibles, heat sinks, mold liners and electrical insulators.
Recently, interest has arisen in inorganic ceramic/organic polymer composites containing BN powders for thermal management applications. It has been suggested in the art that a spherical morphology BN powder would be useful to enhance powder processing of polymers. However, a commercial source of such powders is not available. One known process to obtain small, laboratory-scale samples of spheroidal BN involves reacting trichloroborazine with an aminosilane to form a polymer, poly(borazinylamine), that dissolves in liquid ammonia (NH3). The resulting solution is used to form an aerosol that is passed through a reaction furnace, producing a boron nitride powder composed of primary particles having spherical morphology. Further nitridation in an NH3 atmosphere at a temperature of 1600° C., over a period of time of at least eight hours, gives h-BN particles of overall spheroidal shape with protruding non-uniform blades. This process is not commercially viable since it requires the use of an expensive, commercially unavailable polymer that is made only from an expensive commercially unavailable monomer. (Lindquist, DA et al. Boron Nitride Powders Formed by Aerosol Decomposition of Poly(borazinylamine) Solutions. J. Am. Ceram. Soc. 74 (12) 3126–28, 1991).
As another example, a second method reacts boron trichloride with ammonia, a combination typically used to make platelet morphology h-BN by CVD. The resulting powders are treated at high temperature in a graphite furnace under vacuum. (The patent suggests formation of spherical primary particles although no evidence of the actual morphology is provided.) This process, if successful, is not commercially attractive due to the expense of the starting material, BCl3, and the formation of a corrosive by-product HCl that tends to leave chloride impurities in powders. (EPO No 0 396 448)
A third and potentially more practical approach for the formation of spherical morphology h-BN powders utilizes a process where an aerosol is generated from a saturated (0.9M) aqueous solution of boric acid. The aerosol is passed into a heated tubular reactor where it is nitrided by NH3 in a temperature range of between 600° C. and 1500° C., preferably between 1000° C. and 1200° C. A powder product, BNxOy, is collected that contains significant amounts of oxygen, typically between 40 wt. % to 55 wt. %. The primary particles have spherical particle diameters in the range 0.1 micron to 5 microns. These powders are subsequently nitrided in a second stage in a temperature range of between 1000° C. to 1700° C. under a flowing stream of NH3. The oxygen contents of the resulting boron nitride powders are less than 4 wt. % and the particles retain the spherical morphology (Pruss et al, Aerosol Assisted Vapor Synthesis of Spherical Boron Nitride Powders. Chem. Mater. 12(1), 19–21, 2000; U.S. Pat. No. 6,348,179 to Pruss et al.)
Although the process described by Pruss et al. is practically useful for the production of spherical morphology BN powders, it possesses several drawbacks, including: (a) large amounts of water are injected into the tubular reaction zone in the form of aerosol droplets thereby diluting the NH3 reactant that is required for nitridation of H3BO3 dissolved in the aqueous aerosol droplets, (b) the large amounts of injected water act as a back-reactant with BNxOy aerosol powders; (c) water is also formed as a reaction byproduct in the first stage aerosol nitridation; (d) the BNxOy powders formed in the first-stage nitridation reaction contain large amounts of oxygen; (e) the large amounts of oxygen are difficult to remove in the second-stage nitridation; and (f) there is significant loss of boron as a volatile component during the nitridation process. FIG. 1 illustrates that large amounts of water are deleterious to the nitridation process. Specifically, as expected, at constant gas flow rates and NH3/N2 ratios, the amount of oxygen present in BNxOy powders decreases with increasing reactor temperature from T=600° C. to 1300° C. However, above T=1300° C., the amount of oxygen in the BNxOy powder dramatically increases as a result of a back-reaction between BNxOy and steam or its thermal decomposition products. Due to such drawbacks, alternative solventless or non-aqueous solvent-based aerosol chemical systems have been sought in the industry.
Very few readily available, inexpensive boron reagents exist that are soluble in a non-aqueous solvent appropriate for aerosol formation or aerosol pyrolysis. Similarly, there are very few inexpensive, liquid-phase boron reagents that might be employed directly without a solvent to generate an aerosol. However, at least one family of boron reagents does exist that is commercially available in large quantities at relatively low cost and is soluble in non-aqueous solvents: trialkoxyboranes or trialkylborates, (RO)3B (e.g., R=Me(CH3), Et(C2H5), Pr(C3H3), Bu(C4H9)). These are free-flowing liquids at 23° C. In addition, there is evidence in the literature that suggests that trialkylborates, (RO)3B, react with the common nitriding reagent ammonia, NH3.
For example, U.S. Pat. No. 2,629,732, discloses that (RO)3B (R=lower mol. wt. alkyl groups, preferably CH3) reacts with NH3 in a 1:1 ratio in the gas phase at normal atmospheric pressure and temperature to give adducts, (RO)3B.NH3. Further, other examples in the literature describe a reaction of (MeO)3B with NH3 that is claimed to form an adduct (MeO)3B.NH3 that sublimes at 45° C. and allegedly is stable to at least 375° C. (Goubeau et al., Z. Anorg. Aligem Chem. 266, 161–174, 1951). Goubeau et al. also describe reactions that employ other reactant ratios which produce complex product mixtures that are not identified. The chemistry is proposed to involve elimination of methanol and dimethyl ether. U.S. Pat. No. 2,824,787 to May et al. claims the formation of BN from pyrolysis of a gas mixture of (MeO)3B and NH3 at a furnace temperature above about 850° C. The resulting product is a white powder containing B, N, O, C, and H in varying amounts depending upon reaction conditions. This powder is then heated in NH3 atmosphere to 900–1100° C. to obtain BN. The '787 patent does not describe the morphology and crystallinity of the BN. However, it is likely that these processes produce BN with the traditional platelet morphology.
Further, in a series of patents, Bienert et al. describe the formation of boron-nitrogen-hydrogen compounds, BN3-xH6-3x, from the reaction of boron halides or boric acid esters with NH3 in a heated gas flow tube held at 200° C. or 500° C. The resulting compounds are claimed to be useful for making detrition-resistant boron nitride pressed bodies, boron nitride powder and semiconduction components. (Bienert et al., Ger. Offen. No. 1,943,581; Ger. Offen. No. 1,943,582; Ger. Offen. No. 2,004,360; U.S. Pat. No. 3,711,594.) Finally, Murakawa et al. describe the use of (EtO)3B in a hot gas stream of air and methane to form B2O3 and C. A powder compact was subsequently heated at 900° C. in N2. It was claimed that h-BN with spherical morphology (ave. diameter, approximately 0 14 micron) formed. (Japanese Patent No. JP60,200,811 to Kokai et al.)
Following from these separate observations, Kroenke, et al. (Organoboron Routes to Boron Nitride), U.S. Provisional Application Ser. No. 60/286,275, (filed Apr. 24, 2001) and (Organoboron Route and Process for Preparation of Boron Nitride), U.S. patent application Ser. No. 10/131/301 (filed Apr. 23, 2002), have shown that liquid (RO)3B reagents, with and without nonaqueous solvents, may be used to form boron containing aerosols which can be efficiently nitrided in an AAVRS process The resulting powders have a spherical morphology and contain boron, nitrogen, oxygen, carbon and hydrogen (designated as BNxOyCz) wherein the oxygen contents are much lower (1–30%) than observed in the aqueous aerosol process described in the Pruss, et al. '179 patent for forming BNxOy powders. Further, the oxygen contents of the powders decrease with increasing reactor temperature from approximately 800° C. to approximately 1700° C. as shown in part in FIG. 2. Further, the BNxOyCz powders with low oxygen contents are readily converted in the first stage reactor system or in a second stage nitridation to BN with oxygen contents of approximately <1% and carbon and hydrogen contents of approximately <0.5%. A further and very important benefit of this precursor system is that the rate of production of BNxOyCz powder is significantly higher (approximately greater than 30 times higher) than observed in the aqueous based boric acid process described in the Pruss, et al. '179 patent Therefore, the process described in U.S. Provisional Application Ser. No. 60/286,275 offers significant benefits over the process described in U.S. Pat. No. 6,348,179.
Despite the promising performance of the trialkyl borate aerosol process, there still remains a need in the art for a process that provides spherical boron nitride powders with lower and/or controllable elemental impurity concentrations that employs boron precursor raw materials that are less expensive and preferably less air and moisture sensitive than the alkyl esters of boric acid, (RO)3B.